Method of forming thin metallic films by cathode disintegration



Patented Jan. 28, 1936 METHOD OF FORMING THIN METALLIC FILMS BY CATHODE DISINTEGRATION Julius von Bosse, Ehrenberg, Bolitz, and Kurt Richter, Leipzig, Germany, assignors to La Dispersion Cathodique, Brussels, Belgium, a corporation of Belgium No Drawing. Application April 12, 1932, Serial No. 604,856. In Germany February 9, 1931 2 Claims. (01. 9170.1)

, heating the electrodes.

The production of thin metallic films by cathode disintegration is known, but the process has only been practically applied to a limited extent, owing to the difiiculties resulting from the heating effect of the current supplied to the electrodes. Frequently the heat generated has an unfavourable influence on the articles .to be coated especially in the case of substances, such as wax, feathers, silk fabrics and the like which are particularly sensitive to heat.

It is also known to cool the electrodes thus enabling substances that are sensitive to heat to be coated with a metallic film while simultaneously accelerating'the coating process. The yield obtained from a given current nevertheless remained relatively low.

Heretofore, however, no acceleration in the cathode disintegration has been obtained by Thus, for example, E. Blechschmidt (Ann. d. Phys. 81, (1926), p 1016) states that the cathode disintegration is not accelerated by heating the electrodes, so long as the heating does not extend beyong the gas space. Such an ecceleration is effected by the present invention whereby the means for forming a thin metallic film comprise heated electrodes of a thickness or greatest cross-sectional dimension is from three-tenths of to approximately equal to the mean free path of the atoms formed by disintegration of the cathode. Preferably the electrodes are formed of thin metal strips wires or the like. The heating of the electrodes may be effected in known manner, e. g. by electric resistance heating, high-frequency heating or the like. More simply the heating may be effected by adapting the cross section of the electrodes to the strength of the dull emission current until the desired rise in temperature of the electrodes is obtained by the bombardment of the ions.

The heated electrodes produce a restricted hot area in their immediate neighbourhood which considerably increases the mean free path of the metal atoms in this area as compared with their mean free path elsewhere. Owing to the increased mean free path in this hot area, the diiiusion of metal atoms back to the cathode is diminished, whilst, at the same time the thermally energized gas molecules owing to their increased tendency to ionization are urged to return to the cathode. Moreover an increase in the intensity of disintegration is obtained as-a result of the unusually small radius of curvature of the surface of the electrodes, and their higher temperature, which ensures that stems which have difiused back to the cathode, are again projected therefrom. The tendency of the thermally energized gas molecules to return to the cathode prevents an excessive amount of heat from passing out of the heated area. According to the invention therefore, a very sensitive article, such as a wax plate, absorbs less heat when thin electrodes at red heat are employed, than when the electrodes are water cooled and of larger diameter, and therefore at a lower temperature. The above effect is not due to thermal vaporization, since intensive disintegration occurs at temperatures at which the electrode substance has no efiective vapour pressure. That the employment of slender electrodes produces a previously unobserved efiect is evident from the following:

It has-hitherto been regarded as a law that increasing the voltage and lowering the pressure increases the disintegration, but it has been shown that this does not apply when slender electrodes are used, but that on the contrary there is an optimum disintegration depending on the diameter of the electrodes and the relative distance between them.

The following examples are given by way of illustration:

Example 1 With a pressure of 0.1 mm. of mercury at which the mean free path of gold atoms is about 0.3 mm., a large number of gold wires, 0.3 mm. in diameter, and spaced at intervals of 20 mm. apart, are employed as electrodes. Using carbon dioxide as the filling gas, the optimum disintegration occurs at 1050 volts. If the pressure be reduced'to 0.03 mm. of mercury, the intensity of disintegration, at 1460 volts, is only half that originally obtained, though the current density remains the same.

Example 2 With a pressure of 0.25 mm. of mercury, at which the mean free path of silver atoms is about 0.4 mm., silver wires with a diameter of 0.23 mm. and spaced at intervals of 40 mm. were employed as the electrodes. Using an atmosphere of hydrogen a high velocity of disintegration was obtained.

Example 3 An acetate film is silvered by using fine silver wires, 0.3 mm. in diameter, as electrodes and hydrogen at a pressure of 0.09 mm. of mercury as the filling gas. At this pressure the mean free path of the silver atoms is about 1 mm. The voltage is 1400,'and the current mA. The film is taken out of the apparatus at the end of 4 minutes and the thickness 01. the layer is found; to be v By wag of comparison, it may be observed that;

' for deposition upon an article toflbe coated without excessive heating of the article, which con-' sists in adjusting the size of the electrodes so Will that the greatest cross-sectional dimension 9! said eiectrodes is from three-tenths of to approximately equal to the mean free path of the atoms formed by disintegration of the cathode;

2.- A process for accelerating the cathodic disintegration oi, metallic electrodes heated to red heat in a. vacuum for deposition upon article to be coated without excessive heating of the article, which consists in adjusting the size of the electrodes so that the greatest; cross-sectional dimension of said electrodes islfrom three-tenths of to approximately equal to the mean free path of the atoms formed by disintegration of the cathode.

fi KURT RICHTER.

JULIUS v. BOSSE. 15 

